In mobile communication systems where mobile stations are free to move within the system area and access the system via a base station currently selected by a respective mobile station, the aim is to maintain data concerning the location area of the mobile stations by means of their registrations, so that the system is able to route calls and offer other services to the mobile stations. The pan-European digital mobile communication system GSM has been designed to allow the users to move very widely, both in the network of the home operator and in the networks of other operators in the same country or in other countries. This means that a mobile subscriber may fully access services in many countries by means of a single subscriber contract. Such ability to move inside a mobile communication network or between different mobile communication networks is called roaming. In this connection, the mobile communication networks of different operators are called public land mobile networks (PLMN). Typically a PLMN is limited to one particular country, in addition to which each country may comprise a plurality of mobile communication networks whose coverage areas may be partly or completely overlapping.
FIG. 1 in the attached drawing shows a simplified example of the structure of the GSM mobile communication system as a block diagram. The units shown in FIG. 1 also exist in other mobile communication networks, but different terms may be used for them. A mobile station MS in standby or active mode is continuously in a radio connection with one base transceiver station (BTS), in the case of FIG. 1 to the base station BTS1. The base station system (BSS) consists of a base station controller (BSC) and base stations BTS under its control. Usually several base station controllers BSC are under the control of a mobile services switching centre (MSC). A mobile switching centre MSC communicates with other mobile switching centres, and via a gateway mobile services switching centre (GMSC) with a public telephone network. Location data and other subscriber data concerning a mobile station MS are permanently stored in the home location register (HLR) of the system and temporarily in the visitor location register (VLR) in whose area the mobile station MS is currently located. The location data concerning a mobile station MS is stored in the visitor location register (VLR) with an accuracy of a location area (LA). The visitor location register (VLR) comprises necessary subscriber data concerning all mobile stations MS located in the area of the VLR.
The geographical area controlled by a visitor location register VLR is divided into one or more location areas (LA) within each of which a MS may move freely without notifying the VLR. One or more base stations BTS may operate in each location area. Base stations BTS continuously broadcast information concerning themselves and their surroundings, such as the base station identity code (BSIC), information on adjacent cells, and the location area identifier (LAI). On the basis of the latter a mobile station MS locked to the base station BTS knows in which location area LA it is currently located. If the mobile station MS notices, when changing base stations BTS, that the location area identifier LAI has changed, the mobile station sends a location updating request to the network. Location updating may be rejected e.g. if the mobile station subscriber has no respective to access services in the VLR area concerned. This rejection may result e.g. from the VLR belonging to a PLMN network with which the home PLMN operator of the mobile station MS has no roaming agreement. If the mobile station MS has no access respective in the network, a message is sent to the mobile station MS notifying of the rejection to access the network and also of failed location updating. Otherwise, the location area of the mobile station MS is updated in the visitor location register VLR in whose area the mobile station is currently located. When the mobile station MS moves to the area of a new visitor location register VLR, the subscriber data concerning the mobile station MS is registered as a visiting subscriber in the new VLR and at the same time the subscriber data concerning the mobile station MS is deleted from the VLR of the previous location area. Information on the VLR in whose area the MS is located is forwarded to the home location register HLR.
The problem in the above registration of a mobile station in connection with location updating is that the visitor location register is loaded by the registration of all mobile station subscribers arriving at the area of the visitor location register and having access rights to the network. When the storage capacity of the visitor location register is used, no new mobile station subscriber registrations can be accepted, and thus further location updates have to be rejected. If an area comprises many visiting subscribers from other networks, for example, the capacity of the network does not necessarily remain sufficient to serve its own subscribers.